This section is intended to introduce the reader to various aspects of art, which may be associated with exemplary embodiments of the present invention, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with information to facilitate a better understanding of particular techniques of the present invention. Accordingly, it should be understood that these statements are to be read in this light, and not necessarily as admissions of prior art.
Most gas compressor applications or gas processing facilities today utilize some compressed or processed gas to supply seals, fuel systems and other auxiliary equipment within the process as a utility gas fluid. Most of these auxiliary systems or other uses of the gas being compressed often require the gas to be clean and dry including no liquid condensation during pressure regulation or pressure drop through the seals or auxiliary systems (dew point control). They also often require detoxification and corrosion protection by removing specific hazardous or corrosive gases and liquids like hydrogen sulfide (H2S) or carbon dioxide (CO2) and water.
Traditional methods to strip out these unwanted components require capital intensive and large complex equipment such as molecular sieves, distillation towers, glycol contactor and other traditional separation equipment. The volume of gas required for these utilities is often small relative to the overall gas being handled in the process and if these processes are not required for the entire gas stream the costs associated with this separation, dehydration, detoxification, corrosion control or component selection can be prohibitive to a project, especially in remote locations or locations where infrastructure does not yet exist. Alternative systems to pipe in fuel gas, produce inert gas, supply clean dry seal gas at sufficient pressure can also be very complex and costly.
Compressor shafts are typically sealed using dry gas seals (DGS) which utilize the principle of sealing between a stationary face against a rotating face by using a gas fluid film. This “seal gas” provides the lubrication and cooling properties needed by the seal for long and reliable operation. Seal gas must be free of particulates, free of liquids, and not have physical properties that cause condensation of the seal gas when expanded across the seal faces. A common cause of compressor failure or trip is a result of seals failing and the most predominant cause for seal failure is caused by contaminants in the process gas, both liquid and solid contaminants.
The source of seal gas for many compressor applications is the process gas being compressed. The pressure needed for seal gas is greater than the compressor suction pressure, but less than the compressor discharge pressure. Therefore many applications utilize discharge gas as the seal gas source when suitable. However, in some applications discharge stream components will condense across the seal faces even after filtering and heating.
In some situations such as high pressure sour gas service, the seal gas has been obtained from another utility source such as a fuel gas system. Such gas from the other utility gas source is then compressed and used as seal gas. Such gas is used in order to avoid the liquid contamination or liquid drop out encountered by using the process gas. This requires additional process and separation units to generate the fuel gas and a separate seal gas booster compressor (e.g. a reciprocating compressor), which can itself be a source of oil and particulate contamination. Usually a reciprocating compressor is used for this service due to the high compression ratios and low flows. Reciprocating compressors of this type are usually lubricated with cylinder oil that has some miscibility with the gas, especially at high pressures. Thus it can not be filtered out at high pressure but condenses or “drops out” of the gas when the pressure is dropped through the seals or at pressure regulators that control the pressure to the seals. This cylinder oil “carry-over” into the seal gas may damage and cause premature failure of standard DGS's.
It is also common with high pressure hydrogen compressors in refineries that process gases can have liquids condense out of the gas with the pressure drop across the dry seal faces. An alternative gas sometimes used is hydrogen from a hydrogen make up line from a reciprocating compressor which may also contaminate the gas with lubrication oil.
Another example of a use of the gas being processed or compressed is fuel gas for gas turbines and steam boilers. Modern gas combustors, and low emissions combustors in particular require a substantially constant composition in order to maintain an acceptable operating condition. Additionally, if liquids are entrained or condense (drop) out in these fuel gas systems during pressure drops (e.g. across a fuel control valve) or cooling in piping, problems can result within the turbine or boiler combustion chambers including unstable operation, inefficient operation, reduced reliability, and/or increased emissions of environmentally regulated species, including, for example, nitrogen oxides (NOx), carbon monoxide (CO) and/or sulfur (e.g. sulfur oxides (SOx)) emissions. In addition toxic or corrosive components in these gases can be a safety issue as well as detrimental to the equipment and the environment. Detoxification and corrosion control are described in more detail below.
The removal of H2S, CO2, water and other toxic or corrosive components (such as sulfur containing materials) from a gas stream in order to make it less corrosive or toxic are common challenges in trying to utilize well stream gas or saturated gas as a utility fluid. Removal of these types of components can make seal gas systems and fuel gas systems safer, more reliable and more environmentally friendly or make the utility systems simpler and less costly. Gas processing and drying equipment to condition the gas to remove these toxic or corrosive components can be very costly and complex and are often not feasible for the volumes required for fuel gas or for gas seals in a given process or operation.
Nitrogen or inert gas systems are often used as a utility in gas processing and compression equipment. For example a blanket or inert gas purge is used in seals to ensure toxic or hydrocarbon gasses do not leak to the environment, to prevent an explosive mixture, to sweep out left over hydrocarbons before maintenance, or as a separation barrier between different fluids such as process gas and the lubrication oil in gas seals. Nitrogen systems designed to separate the nitrogen from air are commonly used to provide this inert utility fluid. In some cases the nitrogen is separated out of the process gas if the gas has a high percentage of nitrogen, making it less valuable as a fuel and thus justifying the added high processing cost. However, for small volumes or where the percentage of nitrogen in the gas is small, these types of systems are not justifiable.
New methods of treating process gas for use as a utility gas are needed.